Pore‐Scale Determination of Residual Gas Remobilization and Critical Saturation in Geological CO 2 Storage: A Pore‐Network Modeling Approach

Abstract

Abstract Remobilization of residually trapped CO 2 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 CO 2 injection has ceased. This phenomenon affects the long‐term stability of CO 2 residual trapping and should therefore be considered for an accurate assessment of CO 2 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 CO 2 is delayed. The results, consistent with our previous analysis of the field‐scale Heletz experiments, have important implications for underground gas and CO 2 storage. In the context of CO 2 storage, they provide important insights into the fate of residual trapping in both the short and long term.

Key Points Remobilization of residually trapped gas occurs at a higher saturation than residual saturation, called the critical gas saturation Ostwald ripening tends to slightly increase the mobilization saturation The relative permeability of remobilized gas is significantly reduced, which adds to the security of residual trapping