Numerical Simulation of Casing Failure Induced by Mudstone Hydration: Strain Evolution and Localization Pattern

Mudstone hydration is a critical factor contributing to casing failure during water injection operations in oilfield development. However, the relationship between hydration-induced stress evolution and casing failure mechanisms remains insufficiently understood. In this study, a three-dimensional fluid–solid coupling numerical model of the casing–cement sheath-formation system was developed, based on the nonlinear correlation between mudstone water content and its mechanical properties. Experiment results indicate that increasing the water content from 2% to 8% reduces the shear strength of mudstone by 89.7%. The axial shear strain distribution along the casing exhibits a pattern of lowvalues in the central section and high values at both ends. With increasing water content, the shear strain response evolves through three stages: weak localization at 2–3%, enhanced localization at 4–5%, and pronounced ring-shaped localization at 8%. Furthermore, localization analysis identifies lithological interfaces as high-risk zones, where strain gradient reversals occur within both the cement sheath and casing. By integrating simulation with observed casing failure data, this study elucidates the progression of casing failure mechanisms in the studied oilfield, providing a mechanistic basis for identifying and mitigating casing failure in waterflood-developed oilfields with extensive mudstone intervals and complex geology.