Pore-Scale Determination of Residual Gas Remobilization and Critical Saturation in Geological CO₂ Storage: A Pore-Network Modeling Approach

Remobilization of residually trapped CO₂ can occur under pressure depletion, caused by any sort of leakage, brine extraction for pressure maintenance purposes, or simply by near wellbore pressure dissipation once CO₂ injection has ceased. This phenomenon affects the long-term stability of CO₂ residual trapping and should therefore be considered for an accurate assessment of CO₂ storage security. In this study, pore-network modeling is performed to understand the relevant physics of remobilization. Gas remobilization occurs at a higher gas saturation than the residual saturation, the so-called critical saturation; the difference is called the mobilization saturation, a parameter that is a function of the network properties and the mechanisms involved. Regardless of the network type and properties, Ostwald ripening tends to slightly increase the mobilization saturation, thereby enhancing the security of residual trapping. Moreover, significant hysteresis and reduction in gas relative permeability is observed, implying slow reconnection of the trapped gas clusters. These observations are safety enhancing features, due to which the remobilization of residual CO₂ is delayed. The results, consistent with our previous analysis of the field-scale Heletz experiments, have important implications for underground gas and CO₂ storage. In the context of CO₂ storage, they provide important insights into the fate of residual trapping in both the short and long term.