Wellbore Characteristics that Control Debonding Initiation and Microannuli Width in Finite Element Simulations

Environmental protection is an increasing concern for plugged and abandoned wells within the Gulf of Mexico. Leakage, however, has been shown to occur before the plug and abandonment (P&A) process takes place. The cement used when completing a well may develop leakage paths in the form of microannuli gaps between the casing and cement due to decreases in pressure and/or temperature. The goal of this paper is to use Finite Element Models (FEM) to determine the decrease in pressure or temperature required to initiate debonding of the cement sheath and the corresponding microannuli widths. Results of this paper show that there is a reduction threshold in which microannuli gaps start to initiate and increase in magnitude. A parametric analysis of the parameters within a wellbore has been performed and determined that the dimensions of the wellbore play a critical factor in microannuli gap width. With this knowledge, microannuli gap prediction graphs can be developed for different wellbore dimensions with pressure and temperature variations to determine whether a wellbore is at risk for leakage prior to P&A.