CH ₄ –CO ₂ Hydrate Replacement Outside Liquid CO ₂ Hydrate Stability Zone

The replacement of CH4 with CO2 in natural gas hydrate offers a promising strategy for concurrent energy recovery and carbon sequestration. However, the conventional replacement theories that rely on the superior stability of pure CO2 hydrate are challenged under the high-temperature conditions prevailing in marine hydrate reservoirs, which remain largely unexplored experimentally. This study presents a systematic experimental investigation into the CH4–CO2 replacement process using liquid CO2 at 10 MPa, under thermodynamic conditions (281.2 and 285.2 K) that bracket the stability boundary of pure CO2 hydrate. The impacts of initial hydrate saturation (14%–49%) and free water saturation (1%–35%) are examined. The results reveal that replacement is feasible even outside the pure CO2 hydrate stability zone. The process is initiated not by the relative stability of pure-component hydrate, but by CH4 dissolution into the liquid CO2 phase, shifting the local fluid composition into the mixed-hydrate stability field. The overall process is governed by a kinetic competition between two pathways: the desired guest exchange and an adverse new hydrate formation. The availability of free water is the decisive factor controlling this competition; high water saturation triggers dominant new hydrate formation, causing pore plugging and a drastic reduction in replacement efficiency, ultimately manifesting as an extremely high CO2 storage ratio but with minimal CH4 yield. Conversely, elevated temperature enhances replacement by kinetically alleviating mass transfer limitations. High hydrate saturation inhibits efficiency, causing a power-law decay in performance due to the formation of a more extensive diffusion barrier, revealing a critical trade-off between replacement efficiency and total CH4 yield. These findings provide new mechanistic insights for achieving efficient production in high-temperature gas hydrate reservoirs.