Migration behavior of supercritical and liquid CO₂ in a stratified system: Experiments and numerical simulations

Multiple scenarios of upward CO₂ migration driven by both injection-induced pressure and buoyancy force were investigated in a horizontally and vertically stratified core utilizing a core-flooding system with a 2-D X-ray scanner. Two reservoir-type scenarios were considered: (1) the terrestrial reservoir scenario (10 MPa and 50°C), where CO₂ exists in a supercritical state and (2) the deep-sea sediment reservoir scenario (28 MPa and 25°C), where CO₂ is stored in the liquid phase. The core-flooding experiments showed a 36% increase in migration rate in the vertical core setting compared with the horizontal setting, indicating the significance of the buoyancy force under the terrestrial reservoir scenario. Under both reservoir conditions, the injected CO₂ tended to find a preferential flow path (low capillary entry pressure and high-permeability (high-k) path) and bypass the unfavorable pathways, leaving low CO₂ saturation in the low-permeability (low-k) layers. No distinctive fingering was observed as the CO₂ moved upward, and the CO₂ movement was primarily controlled by media heterogeneity. The CO₂ saturation in the low-k layers exhibited a more sensitive response to injection rates, implying that the increase in CO₂ injection rates could be more effective in terms of storage capacity in the low-k layers in a stratified reservoir. Under the deep-sea sediment condition, the storage potential of liquid CO₂ was more than twice as high as that of supercritical CO₂ under the terrestrial reservoir scenario. In the end, multiphase transport simulations were conducted to assess the effects of heterogeneity on the spatial variation of pressure buildup, CO₂ saturation, and CO₂ flux. Finally, we showed that a high gravity number (Ngr) tended to be more influenced by the heterogeneity of the porous media.