ABSTRACT: The integrity of the cement sheath is the key part to maintain zonal isolation and prevent the inter-zonal communication. Loads arising from multiple stages of wellbore life span may induce various modes of cement failure within the cement (disking and radial cracks) and at the cement-casing and the cement-formation interfaces (debonding fractures). Micro-annuli (MA) are the systematic and inter-connected debonding fractures which is the most hazardous mode of cement failure and can cause serious wellbore leakage problems. This paper utilizes the extended finite element method (XFEM) to study the cement failure under various loading conditions (i.e. cement volume change during hardening, mechanical loads due to borehole pressure change) and investigate the influence of the cement failure to the MA generation during the latter operation stages. A staged 3D finite element analysis approach including loads from various operation procedures during the life cycle of a composite wellbore system is used to establish an in-situ downhole condition and study the conditions of MA generation and evolution. Modeling results indicate that radial cracks are likely to occur during the cement volume shrinkage during the cement hardening and MA (debonding fractures) tend to occur under the periodically cooling during the injection stage. The results also show that the initial stress state in the cement for each procedure is a key factor determining the initiation of different cement failure types. In summary, the more compressive the cement state of stress, the lower the likelihood for radial cracks to initiate and the more likely debonding occurs during thermal cycling. The results with respect to varying cement Young’s modulus show that a high Young’s modulus promotes the initiation of radial cracks. The initiation of interface debonding is independent of cement Young’s modulus. The results presented indicate that a cement system with a low Young’s modulus and high tensile strength provides favorable conditions to promote the cement sheath integrity.
Inter-zonal communication is the unwanted fluid migration along the wellbore through fractures and cracks within the formation rock and cement, and along the composite system interfaces (Parcevaux and Sault, 1984; Dusseault et al., 2000; Nelson and Gilliot, 2004). Zonal isolation is maintained by the mechanical integrity of the wellbore system, especially the integrity of the cement sheath. Various factors during the life cycle of the wellbore (i.e. drilling, completion, injection/production, and well abandonment) may damage the cement sheath and compromise wellbore integrity. Therefore, understanding the occurrence of cement failure under different loading conditions during the life cycle of the wellbore is important in order to predict potential hazards and optimize operation procedures.