Failure Mechanism of Zonal Isolation for Cement Sheath Induced by Density Reduction Operations after Liner Casing Cementing

Zonal isolation in deep natural gas wells is often jeopardized by the formation of microgaps at the cement sheath interface during operations involving significant reductions in wellbore fluid density, which can result in annular gas migration and compromised sealing integrity. This study investigates the bonding strength at the cement sheath interface by establishing casing-cement-formation composite models. The impact of fluid density reduction on cement sheath stress was analyzed across the casing overlap section, open-hole section, and well bottom, revealing depth-dependent variations. The results indicate that greater decreases in fluid pressure lead to larger microannuli, with the microgap size increasing as the depth approaches the well bottom under uniform cement stone properties. For example, at the well bottom, stress changes of up to 45 MPa corresponded to microannuli sizes of 0.058 mm. Enhanced cement strength or a reduced elastic modulus can significantly mitigate the formation of microgaps. In contrast, higher casing eccentricity exacerbates this issue, particularly in narrow annular spaces. This work highlights the novel quantitative evaluation of mechanical parameters and casing eccentricity on microannuli formation, offering theoretical and technical insights for optimizing wellbore density reduction processes. These findings provide critical guidance for improving cement sheath sealing integrity and ensuring reliable zonal isolation in deep wells.