Investigation of CH4 hydrate formation and subsequent phase-dependent CO2 replacement kinetics in coarse porous media

The replacement of CH 4 hydrates with CO 2 is a promising strategy for synergistic energy recovery and carbon sequestration; however, the influence of the physical state of injected CO 2 within high-permeability sediments remains poorly understood. This study comparatively investigates gaseous and liquid CO 2 injection into a coarse quartz sand matrix (711 µm). The results reveal an unexpected kinetic bottleneck for the liquid phase: despite the wide-pore structure that minimizes hydrodynamic resistance, CO 2 liquid achieved a replacement efficiency of only 8.47%, compared to a maximum of 63.65% for CO 2 gas. We identify that the low efficiency of CO 2 liquid is not due to macroscopic flow limitations, but rather an "interfacial armoring" effect. In this mechanism, a dense CO 2 hydrate film forms rapidly at the interface, blocking molecular diffusion a process exacerbated by the specific thermal energy demands and higher specific heat of the liquid phase. These findings demonstrate that high sediment permeability does not compensate for the thermodynamic constraints of liquid CO 2 , redefining site selection and phase criteria for large-scale carbon sequestration in continental margins.