Defect-composition coupling governing mechanical failure in CO₂-CH₄hydrates

Understanding the mechanical stability of CO₂-CH₄heteroclathrate hydrates is essential for assessing reservoir stability during CO₂-CH₄replacement. However, the coupled effects of mixed guest compositions and intrinsic lattice defects on hydrate mechanics remain unclear. In this work, molecular dynamics simulations were employed to investigate the mechanical behavior and microstructural evolution of CO₂-CH₄heteroclathrate hydrates containing controlled water-vacancy defects (0%-2.17%) under uniaxial tension. The results show that increasing CO₂content and water-vacancy concentration both significantly degrade tensile strength, critical strain, and Young's modulus. Mechanical weakening is particularly pronounced when CO₂occupies small 5¹²cages. Water-vacancy defects further accelerate failure by disrupting hydrogen-bond networks, promoting stress localization, and facilitating cage dissociation. Microscopic analyses reveal progressive breakdown of conventional cages and the formation of unconventional cages as transient intermediates, directly linking microstructural degradation to macroscopic softening. A random forest machine learning model was developed to capture the structure-property relationships using defect characteristics, cage statistics, hydrogen-bond evolution, and radial distribution function features. The model achieves high predictive accuracy with errors within 5%, and feature-importance analysis highlights the dominant role of microstructural descriptors in governing different mechanical properties. These findings provide atomistic insights into defect-composition coupling effects and establish a predictive framework for evaluating hydrate mechanical stability during CO₂-CH₄replacement and geological storage.