Molecular Insights into CO 2 –N 2 Coinjection-Driven Methane-Hydrate Replacement and CO 2 Sequestration in Clay Nanopores

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