Calculation of micro-annulus size in casing-cement sheath-formation system under continuous internal casing pressure change

The change of internal casing pressure could result in the failure of the cement sheath or the occurrence of the micro-annulus at the interface between the cement and the casing, or at the interface between the cement and the formation due to plastic deformation of the cement. In the hope to estimate the size of the micro-annulus, a theoretical model based on Mohr-Coulomb yield criterion which considers the interaction among casing, cement sheath and formation is built. The plastic behavior of the cement sheath and the bonding strengths at two interfaces are taken into account. Particularly, the initiation and the development of micro-annulus at two interfaces (casing-cement sheath interface and cement sheath-formation interface) are analyzed. This model is further used to simulate Jackson and Murphey's experiment, which studied the gas channeling in the annular space. Good agreement is found in general. Results show that both loading and unloading processes contribute to the initiation of the micro-annulus. Loading process may lead the cement sheath into plasticity. The radial stress at the interfaces turns into tensile stress during the unloading process. Micro-annulus could appear at both interfaces if the tensile stress exceeds the corresponding bonding strengths. If the bonding strengths at two interfaces are of similar magnitude, the micro-annulus would be more likely to appear at the casing-cement sheath interface due to its higher tensile stress. This model can be used to evaluate the risk of cement sealing failure, especially during the hydraulic fracturing, and lower the risk of zonal isolation failure.