Percolation transitions of spontaneous imbibition in fractional-wet porous media

Abstract

Percolation transition is an important scientific question for characterizing complex two-phase flow phenomena in fractional-wet porous media composed of different wetting grains. Recent studies have shown significant transitions of fluid distribution and pressure system in fractional-wet porous media by changing wettability proportion. However, very few studies have systematically investigated controlling factors of percolation transition in spontaneous imbibition. Additionally, there is a lack of comprehensive knowledge and experimental observations about imbibition recovery and dynamic imbibition phenomena (including imbibition time, imbibition rate, and induction time) in fractional-wet porous media. In this study, these knowledge gaps are filled through a series of water imbibition experiments for fractional-wet artificial porous media with different water-wet proportions, three pore diameters, and two wettability heterogeneities. The results show that pore diameter and wettability heterogeneity are influential factors for percolation transition. Decreasing water-wet proportion results in the emergence of induction time, as well as the absences of transition and late imbibition stage, small pore radius enhances these negative impacts. Under strong wettability heterogeneity, relationship of water-wet proportion and imbibition recovery follows the classical percolation theory containing two percolation transitions. Small pore diameter retards percolation transitions, and weakens fluid displacements of the transition stage and late stage in the partial imbibition. Moreover, effects of water-wet proportion on dynamic imbibition phenomena exhibit exponential relationship resulted from transition of pathway type. Additionally, through decreasing wettability heterogeneity, three percolation transitions and an optimal imbibition ability are achieved, thus hydrophilization of hydrophobic pore is advantageous for water imbibition. These intriguing findings can broaden current knowledge about spontaneous imbibition in fractional-wet porous media.