Through-tubing, cased-hole sealed material evaluation using acoustic measurements

A system comprising:

• an emitter to be positioned within a production tubing located in a casing of a wellbore that includes a sealing material positioned between the casing and a subsurface formation in which the wellbore is formed, wherein the emitter is to emit an acoustic waveform outward toward the subsurface formation;
• a detector that is to detect a return waveform generated in response to the acoustic waveform interacting with a region of interest of the sealing material in an annulus between the casing and the subsurface formation;
• a processor;
• and
• a computer-readable medium having instructions stored thereon that are executable by the processor to cause the system to,
• determine a first time window associated of the returned waveform associated with the region of interest, wherein determining the first time window comprises:
• defining a virtual interface within the subsurface formation;
• determining a maximum time for the return waveform based on a distance between the detector and the virtual interface;
• determining an out-of-interest time based on the maximum time and a geometry of the wellbore;
• and
• determining an offset time for the return waveform based on a property of the casing, wherein the first time window is based on the offset time and the out-of-interest time;
• trim the return waveform based on the first time window;
• determine a first spectral power density for the first time window of the trimmed return waveform;
• and
• determine a composition ratio value for the region of interest based on the first spectral power density.

A non-transitory, computer-readable medium having instructions stored thereon that are executable by a processor to perform operations comprising:

• determining a first time window of a return waveform associated with a region of interest that includes at least a portion of a sealing material positioned between a casing of a wellbore and a subsurface formation in which the wellbore is formed, wherein the return waveform is generated in response to an acoustic waveform being emitted from a position within a production tubing that is within the casing, and
• wherein determining the first time window comprises:
• defining a virtual interface within the subsurface formation,
• determining a maximum time for the return waveform based on a distance between the detector and the virtual interface,
• determining an out-of-interest time based on the maximum time and a geometry of the wellbore, and
• determining an offset time for the return waveform based on a property of the casing, wherein the first time window is based on the offset time and the out-of-interest time;
• trimming the return waveform based on the first time window;
• determining a first spectral power density of the trimmed return waveform;
• and
• determining a composition ratio value for the region of interest based on the first spectral power density.