Pore-scale modeling of water–gas flow in heterogeneous porous media

Water–gas flow in heterogeneous porous media is a ubiquitous natural phenomenon. A pore-scale investigation can help to understand the mechanisms of water–gas flow. This study employs a direct simulation method to model the immiscible water–gas flow while tracking the phase interface via the phase-field method. We first verified the mathematical model by layered two-phase flow and capillary intrusion tests. Then, the quartet structure generation set was used to generate a heterogeneous porous media, based on which water–gas displacement was simulated. The characteristics of drainage and imbibition displacements were systematically investigated. Results show that the forced imbibition process shows stable displacement due to cooperative filling, yet with local capillary fingering. Capillary valve effects always exist during the process, making the capillary force act as both driving and resistance forces in heterogeneous porous media. Nevertheless, these pore-scale events inhabit the rapid breakthrough in the small pore-throat zone, ensuring the uniform advancement of the interface. During drainage, viscous fingering in the wide pore-throat zone and capillary fingering in the narrow pore-throat zone are simultaneously observed. Compared with the imbibition process, the water–gas front advances faster due to the smaller viscous force of invading fluid. The phase distribution after drainage displacement at different capillary numbers is quite different due to inconsistent flow patterns. Nevertheless, the final phase saturation of the imbibition process under different capillary numbers is similar, but the area of each type is different. For both the imbibition and drainage processes, the larger the capillary number, the higher the final displacement efficiency.