Microannulus
Microannulus is a very small gap that can form between the casing or liner and the surrounding cement in primary cementing operations. Microannulus can jeopardize the hydraulic efficiency of a primary cementing operation and act as a channel for reservoir fluid migration, compromising the well's integrity. Microannulus could be created due to temperature & pressure variations during or after the cement job. These variations can cause cement shrinkage, cracking, or reverse ballooning of casing to create these micro gaps. Improper removal of mud or film of oil on the pipe before displacing with cement can also result in microannulus.