The injection of CO₂ into CH₄ hydrate-bearing sediments causes the release of CH₄ and the formation of CO₂ hydrate within the CH₄ hydrate stability field. CH₄-CO₂  replacement allows for the recovery of an energy source, CH₄, while trapping CO₂. In this study, we monitor pore-scale changes in electrical resistance and relative stiffness during CH₄ hydrate formation, CH₄-CO₂ replacement, and hydrate dissociation; experiments are also observed using high-resolution time-lapsed photography. Results show that CH₄-CO₂ replacement occurs locally and gradually so that the overall hydrate mass remains solid and no stiffness loss should be expected at the sediment scale. Other experimental results confirm the slow diffusion of CH4 through the hydrate shell that forms between water and gas; this may allow for the coexistence of gas-hydrate-water phases for long periods of time.