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.