Determining residual gas remobilization and critical saturation in geological CO2 storage by pore-scale modelling

Remobilization of residually trapped CO2 as a result of pressure depletion occurs inherently at the pore-scale but affects the long-term stability of the residual trapping of CO2 at larger scales. In this study, pore-network modelling (PNM) is used to investigate this phenomenon under pressure depletion conditions. 3D networks of Bentheimer and Heletz sandstone as well as statistically generated generic 2D and 3D networks are used. The gas remobilization does occur at a higher gas saturation than residual saturation, so-called critical saturation. The difference is denoted as mobilization saturation, which varies according to the network properties (e.g., dimensionality) and the processes/mechanisms involved. Slightly smaller values are obtained for 3D networks due to the higher order of geometric connectivity between the pores and the effects of gravity. Regardless of the network types and properties, Ostwald ripening tends to slightly increase the mobilization saturation, thereby enhancing the security of residual trapping. Moreover, a significant hysteresis and reduction in gas relative permeability is observed during the depletion process, implying slow reconnection of the trapped gas clusters. These observations are safety enhancing features, due to which the remobilization of the residual trapped CO2 is delayed. The results, which are consistent with our previous analysis of field-scale Heletz experiments, have important implications for underground gas and CO2 storage. In the context of geological CO2 storage, they provide important insights into the fate of residual trapping in both the short and long term.