A micro-annulus refers to a minimal gap/clearance mostly at the interface between the cement and casing pipe (see Fig. 6.1a), though the topic may be sometimes generalized to the interface between the cement and wellbore/formation (Fig. 6.1b). Generally, a micro-annulus develops due to pressure and temperature changes in the wellbore. A micro-annulus aperture size/thickness typically ranges from a few to a few hundred microns (Issabekov et al., 2017). The aperture may reach up to 50 μm for a micro-annulus due to thermal debonding, and from 50 to 300 μm due to mechanical debonding (Fernandez et al., 2019). For size comparison, the average human hair size is 50 μm. The micro-annulus gap may be filled with either liquid (called wet micro-annulus) or empty space/gas (dry).