Trapping and hysteresis in two-phase flow in porous media: A pore-network study

Several models for two-phase flow in porous media identify trapping and connectivityof fluids as an important contribution to macroscale hysteresis. This is especially true forhysteresis in relative permeabilities. The trapping models propose trajectories from theinitial saturation to the end saturation in various ways and are often based on experiments orpore-network model results for the endpoints. However, experimental data or pore-scalemodel results are often not available for the trajectories, that is, the fate of the connectivityof the fluids while saturation changes. Here, using a quasi static pore-network model,supported by a set of pore-scale laboratory experiments, we study how the topology of thefluids changes during drainage and imbibition including first, main and scanning curves. Wefind a strong hysteretic behavior in the relationship between disconnected nonwetting fluidsaturation and the wetting fluid saturation in a water-wet medium. The coalescence of theinvading nonwetting phase with the existing disconnected nonwetting phase dependscritically on the presence (or lack thereof) of connected nonwetting phase at the beginningof the drainage process as well as on the pore geometry. This dependence involves amechanism we refer to as ‘‘reversible corner filling.’’ This mechanism can also be seen inlaboratory experiments in volcanic tuff. The impact of these pore-network model results onexisting macroscopic models is discussed.