Pore-scale visualization on a depressurization-induced CO₂ exsolution

The pore-scale behavior of the exsolved CO₂ phase during the depressurization process in CO₂ geological storage was investigated. The reservoir pressure decreases when the injection stops or when a leaking event or fluid extraction occurs. The exsolution characteristics of CO₂ affect the migration and fate of CO₂ in the storage site significantly. Here, a micromodel experimental system that can accommodate a large pressure variation provides a physical model with homogeneous porous media to dynamically visualize the nucleation and growth of exsolved CO₂ bubbles. The pressure decreased from 9.85 to 3.95MPa at different temperatures and depressurization rates, and the behavior of CO₂ bubbles was recorded. At the pore-scale, the nuclei became observable when the CO₂ phase density was significantly reduced, and the pressure corresponding to this observation was slightly lower than that of the severe expansion pressure region. The lower temperature and faster depressurization rate produced more CO₂ nuclei. The exsolved CO₂ bubble preferentially grew into the pore body instead of the throat. The progress of smaller CO₂ bubbles merging into a larger CO₂ bubble was first captured, which validated the existence of the Ostwald ripening mechanism. The dispersed CO₂ phase after exsolution shows similarity with the residually trapped CO₂. This observation is consistent with the low mobility and high residual trapping ratio of exsolved CO₂ measured in the core-scale measurement, which is considered to be a self-sealing mechanism during depressurization process in CO₂ geological storage.