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 3.95 MPa 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.