Effect of the Immiscible-to-Miscible Transition on Gas Injection Pore Occupancy in Network Model with Various Wettabilities

The underlying physics of three-phase displacements in porous media plays a crucial rule in CO₂ storage processes. A useful step towards a fully understanding of this phenomenon is studying pore occupancy sequences of phase displacements. This is now possible since modern digital rock analysis offers a non-destructive method to view pore occupancies. In this study, a 3D pore network model is used to study pore occupancy sequences of gas injection in two wettability structures (water-wet and oil-wet) at immiscible and near-miscible conditions by applying the extended Bartell-Osterhof theory of van Dijke and Sorbie. Multiple displacements and thermodynamic criteria of oil layer formation and collapse are considered to simulate the full physics of three-phase displacements in capillary dominated systems.

The results show that pore occupancy of gas injection is very similar to the basic theory and depends on the wetting order of the injected phase. For water-wet systems, gas is always the least wetting phase and invades through largest pores under all miscibility conditions, while the situation is different for oil-wet systems as wetting order of the gas changes from the non-wetting to intermediate wetting as miscibility approaches. These findings align closely with pore occupancy sequences observed in micro-CT image experiments.