  Film flow through networks of corners and capillary bridges can establish
connections between seemingly isolated clusters during drainage in porous
media. Coupled with drainage through the bulk of pores and throats, the flow
through these networks constitutes a secondary drainage mechanism that can
significantly affect fluid configurations and residual saturations. In order to
investigate the prevalence of this drainage mechanism, we propose a
quasi-static pore-network model based on modifying the
trapped-cluster-identification algorithm in an invasion-percolation model. With
the modification, wetting-phase connectivity could be provided by direct
successions of pores and throats, represented by sites and bonds, as well as by
chains of interconnected capillary bridges. The advancement of the fluid
interface in the porous matrix was determined by the bonds' invasion
thresholds, calculated based on local capillary pressure values that could be
perturbed to accommodate gravitational effects. With the proposed model,
experimentally verified phenomena related to slow drainage in granular porous
media were reproduced, showing good qualitative agreement.
