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.