Molecular Insights into CO₂-N₂ Coinjection-Driven Methane-Hydrate Replacement and CO₂ Sequestration in Clay Nanopores

Marine methane hydrates combine high energy density with low-carbon attributes; replacing cage-encapsulated CH₄ with CO₂ is regarded as a dual-benefit route that couples production with emissions reduction. Recently, CO₂-N₂ coinjection has been proposed to alleviate the mass-transfer limitations of pure CO₂ replacement, yet its molecular mechanism under clay confinement remains poorly understood. Here, we perform microsecond molecular dynamics simulations to systematically assess how varying N₂ fractions influence CO₂-hydrate growth and CO₂-for-CH₄ replacement. Results show that, under confinement, CO₂ replacement remains constrained by the interfacial hydrogen-bond network. In the absence of N₂, methane hydrate dissociates from crystal edges and eventually vanishes; CH₄/CO₂ bubbles occupy clay adsorption sites, drive Na⁺/Cl^- ions toward the pore center, and suppress secondary nucleation. At low N₂ ratios (N₂/CO₂ = 1:2, 1:3, 1:4), CH₄ escape and hydrate dissociation are markedly reduced, yet the promotion of CO₂/N₂ mixed-hydrate nucleation and growth is limited. In contrast, a higher ratio (e.g., 1:1) stabilizes the pre-existing cage framework, compensates for CO₂'s weak occupancy of 512 cages, improves the hydrogen-bond network and precursor coordination, and advances CO₂/N₂ mixed-hydrate formation, increasing the Na⁺/Cl^- fraction on clay surfaces and reducing ion-perturbed water molecules. Therefore, higher N₂ fractions favor direct CO₂ sequestration, whereas lower fractions are suited to partially controlled dissociation and targeted gas production. These insights provide molecular-scale guidance for optimizing gas ratios and operating windows for CO₂ sequestration in methane-hydrate reservoirs and furnish mechanistic support for cleaner hydrate development.