Methods for determining positions of fluid interfaces and detecting cement setting in a subterranean wellbore

A system, comprising:

• a pump adapted to pump a first fluid and a second fluid from a bottom of an annulus formed between a casing and a borehole toward a top of the annulus;
• a vibration generator installed aboveground directly on the casing, the vibration generator adapted to directly induce physical vibrations along the casing;
• a vibration detector positioned aboveground proximate the casing and adapted to detect oscillations in at least one of the casing, the first fluid, or the second fluid;
• and
• a data acquisition system connected to the vibration detector, the data acquisition system adapted to determine, based on the oscillations, a position of an interface formed between the first fluid and the second fluid in the annulus.

A method of determining when cement has been set in a subterranean well, the method comprising:

• pumping a cement slurry and a second fluid from a bottom of an annulus formed between a casing and a borehole toward a top of the annulus;
• generating, by a vibration generator installed aboveground directly on the casing, direct physical vibrations along the casing;
• detecting, by a vibration detector positioned aboveground proximate the casing, oscillations in at least one of the casing, the cement slurry, or the second fluid;
• and
• determining, based on a frequency shift of the oscillations, whether the cement slurry has set.